1. Field of the Invention
Nuclear power plants are required by practice to be designed in such a manner that the health and safety of the public is assured even for the most adverse accidents that may be postulated. For plants utilizing light water as a coolant, the most adverse accident is considered to be a double-ended break of the largest pipe in the reactor coolant system and is termed, Loss of Coolant Accident (hereinafter sometimes referred to as LOCA).
For accident protection, these plants utilize containment systems that are designed to physically contain water, steam and any entrained fission products that may escape from the reactor coolant system. The containment system is usually considered to encompass all structures, systems and devices that provide ultimate reliability in complete protection for any accident that may occur. Engineered safety systems are specifically designed to mitigate the consequences of an accident. Basically, the design goal of a containment system is that no radioactive material escapes from the nuclear power plant in the event of an accident so that the lives of the surrounding populace are not endangered.
The passive containment system herein disclosed provides this level of protection for a loss of coolant accident and for the other types of accidents that are considered as a basis of design, and is considered to be effective for nuclear power plants employing boiling water reactors.
2. The Prior Art
Prior art techniques have utilized pressure suppression containment for the boiling water reactors. The pressure suppression containment consists of a drywell that houses the reactor coolant system, a pressure suppression chamber containing a pool of water and a vent system connecting the drywell to the pool of water. This containment structure is usually either constructed of steel enclosed by reinforced concrete or is steel-lined with reinforced concrete. The pressure suppression containment is housed within a reactor building.
In the event of a LOCA, the reactor coolant partially flashes to steam within the drywell, and the air, steam and liquid coolant flow through the connecting vents into the pool of water in the suppression chamber. The steam is condensed by the water and decreases the potential pressure rise in the containment. The air rises into the free space above the pool of water in the suppression chamber.
Refinements in pressure suppression containment utilizing water includes the inerting of the containment atmosphere. Inerting is aimed at preventing the burning of hydrogen evolved from metal-water reactions of overheated nuclear fuel.
The pressure-suppression containment is a passive structure that requires support systems for accident containment. Active systems such as residual heat removal systems and spray systems are used to dissipate heat to the environs. This prevents the containment design pressure and temperature from being exceeded and, in the process, the containment pressure is reduced to thereby limit the leakage of fission products. Active filtration systems are at times utilized in conjunction with the spray systems to reduce fission product concentration in the containment atmosphere. This also limits the amount of fission products that can leak out of the containment to the environs. Hydrogen recombiners are also utilized to protect the containment from developing explosive concentration of hydrogen.
To be effective, the pressure suppression containment thus requires active engineered safety systems that provide emergency cooling of the nuclear fuel. Active engineered safety systems are inherently required to function effectively in order to maintain the integrity of the containment system in the LOCA. Active systems require high integrity instrumentation and control equipment, rotating machinery, electric power sources and power distribution equipment. These systems need to function properly as part of a larger system under adverse containment environment conditions of high pressure, high temperature, high humidity, high radioactivity and eroded thermal insulation.
Malfunctioning of any active engineered safety system imposes even more adverse conditions on the operable system. For instance, an inadequate source of electric power may result in the malfunctioning of the emergency core cooling system for the nuclear fuel. Overheating of the fuel can result in melting of the fuel cladding with metal-water reactions occurring. The fuel core may slump and portions could collapse and overheat the bottom of the reactor vessel. Hydrogen is, in turn, released from metal-water reactions and is subject to burning. The added energy from the metal-water reactions and from the burning of hydrogen imposes even more severe requirements on the containment structure. Overheating of the fuel and melting of the clad results in a gross release of fission products that are available for leakage out of the containment system. This example points to the critical nature of active engineered safety systems that are an essential part of the containment system of the prior art.